1. Field of the Invention
The present invention relates to a control device for controlling an operation of an electric power conversion circuit including voltage-driven type power switching elements whose conduction states are controlled by voltages which the control device applies to their conduction control terminals in order to convert an input voltage into a desired output voltage.
2. Description of Related Art
As such an electric power conversion circuit, there is known an inverter having power switching elements operating to convert an input DC voltage into an AC output voltage to be applied to a three-phase electric motor. It is also known that a control device of the inverter operates to control a voltage applied to each phase of the three-phase motor at a desired value by performing a switching control on the power switching elements of the inverter to control a voltage applied to each phase of the motor, as disclosed in Japanese Patent Applications Laid-open No. 9-56195, and No. 2004-222420.
Generally,. as electric power supplied to the motor increases, that is, as the output power of the inverter increases, a power loss of the power switching elements increases. This may cause the temperature of the switching elements to rise beyond their allowable upper limit.
Accordingly, in view of preventing the temperature of the power switching elements from exceeding their allowable upper limit when they are operating at around their maximum power, it is common to upsize the power switching element. This makes it possible to reduce the amount of heat emitted by the power switching elements, to thereby improve the reliability of the inverter.
As explained above, in using the conventional control device for the power conversion circuit such as and inverter having voltage-driven type power switching elements, the size of the circuit and the amount of the emitted heat are in a tradeoff relationship.